The term dead-pressing is commonly used to describe both static and dynamic pressure effects. However, technically dead-pressing applies only to relatively slow or steady static compression effects. These can be produced by a hydraulic press, as in the original meaning, by immersion of the explosive in water-hydrostatic head pressure, application of compressed air, or mechanical "squeezing" due to the mass of rock surrounding an explosive-tectonic pressure. Conversely, dynamic pressurization is a very rapid transient phenomenon induced by shock waves or pressure pulses produced by adjacent bore hole explosions during delay blasting operations. The term "pressure desensitization" is used herein to cover both static and dynamic effects. Both can have the same effect on the explosive-reduced performance or failure.
The term nonincendive explosive means one that will not cause the ignition of flammable atmospheres that can be present in underground mines. A nonincendive explosive can be classified as a Permissible if it passes the required tests and is approved [as per U.S. Code of Federal Regulations (CFR). Title--30 Mineral Resources; Chapter 1--Mine Safety Health Administration (MSHA), Dept. of Labor; Subchapter B--Testing, Evaluation, and Approval of Mining Products, Part 15--Requirements for Approval of Explosives and Sheathed Explosive Units, Jul. 1, 1990, pp. 89-96]. These tests are performed by the US Bureau of Mines. Incendivity--Gallery tests numbers 7 & 8 incorporate flammable atmospheres consisting of 8% natural gas/air and 4% natural gas-coal dust/air mixtures respectively. In test 7, the Bruceton up-and-down statistical method, is used to determine W.sub.50 (charge weight for a 50% probability of ignition) to L.sub.95 (lower limit for 95% confidence). The minimum W.sub.50 passing level is 450 g to 95%. In test 8 the minimum passing level is a W.sub.cdg (weight of explosive in coal dust and gas) of 350 g for which there is a 50% probability of ignition.
The Air-Gap Sensitivity test (explosion by influence test) evaluates the ability of a candidate explosive to propagate detonation across a gap between adjacent explosive cartridges. The minimum allowable air-gap is 7.6 cm (3 in) at an explosive temperature of 25.degree..+-.5.degree. C. (77.degree..+-.9.degree. F.), and 5.1 cm (2 in) at 5.degree. C.
Other tests are also required to establish permissibility.
Several emulsion explosive compositions described in the prior art utilized various types of microspheres/bulking agents such as glass microballoons, plastic microspheres, perlites, expanded minerals, cushioning medium of structural foam, and chemical gassing agents to control the density of the emulsion and also to impart sensitivity. These emulsion explosive formulations disclosed in various patents claim several improvements in performance, such as high stability, good initiation sensitivity, good cold temperature sensitivity, and high detonation pressure. However, these compositions of the prior art do not perform satisfactorily when subjected to high static and/or dynamic pressures. These emulsion explosives often fail to detonate due to dead-pressing phenomenon, caused by shock waves of adjacent bore hole explosions during delay blasting operations.
Most nonincendive (permissible) explosives of prior art add sodium chloride directly, as the primary nonincendive agent. However, the direct addition of sodium chloride (in dissolved or suspended form) to an emulsion explosive can severely reduce the air-gap sensitivity and in some cases make it not cap sensitive.
Previous emulsion explosives added monomethylamine nitrate to improve air-gap sensitivity.
Those concerned with these and other problems recognize the need for an improved high pressure-resistant emulsion explosive composition.